1. Field of the Invention
The present invention relates to a spark plug which is mounted in an internal combustion engine to ignite an air-fuel mixture.
2. Description of the Related Art
Conventionally, a spark plug for ignition is used in an internal combustion engine such as an automobile engine. A spark plug generally includes a center electrode forming a spark discharge gap on its leading end side; an insulator having an axial hole for holding the center electrode; and a metal shell radially surrounding and holding the periphery of the insulator. An external thread is formed on an outer peripheral surface of the metal shell so as to engage an internal thread formed in a mounting hole provided on the engine side for mounting the spark plug. Furthermore, the spark plug is mounted in the engine to ignite the air-fuel mixture by a spark discharge.
In recent years, trends toward higher power and fuel savings in internal combustion engines are underway. Consequently, there has been a demand for a reduction in the diameter of the mounting hole from the viewpoint of ensuring a high degree of freedom in design of the engine, including, for example, enlarging the exhaust valve diameter for improving exhaust efficiency and providing a dense water jacket arrangement for improving cooling efficiency. Accordingly, a spark plug is known in which the thread (whose diameter is difficult to reduce) is disposed at the rear end side of the outer peripheral surface of the metal shell, and the outer peripheral surface on the leading end side is made thread-free (not formed) (e.g., refer to JP-A-2000-504875 (corresponding to U.S. Pat. No. 5,697,334)). In JP-A-2000-504875 (corresponding to U.S. Pat. No. 5,697,334), the leading end (truncated cone portion) of the metal shell is tapered, and a sleeve formed separately and having a small diameter protrudes from and is retained at the leading end in an axially aligned state. In this state, a spark discharge is carried out at the leading end side of the sleeve, thereby realizing a small diameter in the configuration of the leading end side located forward of the metal shell. By so doing, it is possible to reduce the diameter of the portion where the sleeve is disposed within the mounting hole, i.e., the portion which is close to the combustion chamber. It is also possible to enhance the degree of freedom in the layout of parts disposed in its vicinity and to enlarge the diameter of the exhaust valve and to provide a densely arranged water jacket.
Meanwhile, a spark plug is also known in which the thread is disposed at the rear end side of the outer peripheral surface of the metal shell, the outer peripheral surface on the leading end side is made thread-free (no thread is formed on the leading end side), a tapered seal portion (stepped portion) is provided on the leading end side, and the outside diameter of a portion located forwardly of the stepped seal portion is made small (e.g., refer to JP-A-2006-12464). In JP-A-2006-12464, the configuration is such that the seal portion abuts a stepped portion provided on the inner peripheral surface of the mounting hole so as to retain the spark plug and seal the combustion chamber. However, the length of the axial direction of the clearance between the inner peripheral surface of the mounting hole and the outer peripheral surface of the metal shell, as viewed from the combustion chamber, is restricted by bringing the seal portion closer to the combustion chamber. By doing so, it is possible to restrain entry of foreign substances such as combustion gases and soot into the clearance during operation of the internal combustion engine, thereby making it possible to prevent a decline in thermal conductivity and to satisfactorily effect heat dissipation of the spark plug.
3. Problems to be Solved by the Invention
However, in the spark plug disclosed by JP-A-2000-504875 (corresponding to U.S. Pat. No. 5,697,334), since the sleeve and the metal shell are formed separately, there is a possibility of axial misalignment between the axis of the metal shell and the axis of the sleeve. For this reason, if there is too tight of a fit between the inside diameter of the mounting hole and the outside diameter of the sleeve, there are cases where inserting the spark plug becomes difficult at the time of mounting. Consequently, a large clearance between the inside diameter of the mounting hole and the outside diameter of the sleeve is unavoidably adopted. In this manner, foreign substances such as combustion gases and soot may possibly enter the clearance during operation of the internal combustion engine, causing a reduction in thermal conductivity and a buildup of heat in the spark plug.
On the other hand, in the spark plug described in JP-A-2006-12464, since the seal portion is arranged close to the combustion chamber, the portion located rearwardly of the seal portion and having a large outside diameter is also disposed near the combustion chamber. Hence, the arrangement layout of other parts in the vicinity of the spark plug is restricted, leading to a decrease in the degree of freedom in engine design.